The proposed experiments attempt to develop an explicit functional description of the processes underlying light and dark adaptation and also color appearance. An attempt will be made to relate the visual phenomena to underlying physiological and anatomical mechanisms, but emphasis is given to an explicit description of the variables controlling visual sensitivity as a first step in relating these phenomena to their physiological underpinnings. The following specific issues are addressed: (1) what are the precise mechanisms by which lateral inhibitory processes control sensitivity? (2) Can the after effects of bright light exposure and the effects of continuously exposed weaker lights be described in terms of one variable, or are a number of processes involved? (3) What is the role of lateral neural connections in hue constancy?